What are the advantages of turning on "cache BVH"?
From the 2.62 release notes -


  For renders where only the camera or materials are changing, while meshes stay fixed, it is now possible to skip the sometimes slow BVH step, by enabling BVH Cache in the Performance panel. This will store the BVH on disk for subsequent (animation) renders of the same scene. Note that if the scene does change in the animation, enabling this option will make rendering somewhat slower.


Times when it will help -


Test rendering one frame while you adjust materials.
A camera flying through a static scene


Times when it will not help -


Any mesh changes during an animation (shapekeys or armature)
Any object movement during an animation